Method of shell molding



pattern detail.

United States Patent METHOD OF SHELL MOLDING Fred J. Webbere, Royal Oak, Micl1., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application May 9, 1952, Serial N 0. 287,035

11 Claims. (Cl. 22-493) This invention relates to the forming of sand-resin molds for foundry casting operations and particularly to a method of carbonizing the binder in shell-type sandresin molds through the use of a non-oxidizing atmosphere, thereby providing stronger, more permeable molds and sounder castings.

Recently developed techniques in foundry practice incorporate the use of thin-walled dispensable molds and cores composed of sand and plastic binders. This procedure, generally referred to as the shell molding process, is particularly suited to the production of precision or semi-precision castings in a wide variety of metals.

Essentially the shell molding process consists of using a thermosetting plastic or resin as a binder for the sand grains to form thin-walled molds having high gas permeability, good surface smoothness and dimensional stability. The molding material, which is generally a dry mixture of a major proportion of silica sand and a minor proportion of a plastic binder, is used in powder form with no water being added. Phenol formaldehyde and melamine formaldehyde resins are typical examples of the type of thermosetting binders normally used. The sand employed is preferably free of metallic oxides, clay, moisture and organic matter.

These sand-resin molds are prepared by allowing the dry mixture of sand and resin powder to come into contact with a hot metal pattern for a short period of time. A layer of the mix adheres to the metal surfaces due to the melting of the resin which entraps the sand with which it is intimately mixed, thereby reproducing Metal patterns should be employed because they are subjected to elevated temperatures. Pattern temperatures in the range of between 250 F. and 350 F. are typical, but temperatures up to 800 F. may be advantageously employed under particular conditions. Half patterns, gates and runners are usually all permanently fixed on the metal plates. The pattern temperature and the length of time the molding material is allowed to remain in contact with the hot pattern surfaces determine the resulting thickness of each mold. Mold build-up times ranging from a few seconds to approximately one minute are appropriate for various applications.

After this short time interval, the excess dry sand and resin are removed, and the closely adhering sandresin layer may be cured by heating in an air oven for a short period of time while in contact with the metal pattern. This baking operation results in the conversion of the resinous material to a hard, insoluble binder which securely bonds the sand grains together. After the removal of the pattern and mold from the curing ice partial burning out of the resin binder. Moreover, difficulties hereto-fore have been encountered in casting certain alloys in shell-type sand-resin molds due to the reaction of the molten alloys with the mold material and/ or volatile products derived therefrom. As a typical example, readily oxidizable metals, such as certain nickel base alloys containing aluminum and titanium, have proved particularly diflicult to properly cast in these sand-resin molds, which may contain between 2% and 25% by weight of a thermosetting resin binder.

Upon contact with the molten metal, the resin binders of the shell molds give 013? a large volume of volatiles, this being evidenced by the considerable amount of flame arising from the molds during and after the pour and the quantity of the resultant condensate. The oxidizing gases which are given off by the resin binders react with many metals during pouring operations to form oxide films which fold into the surface of the metal, re sulting, in most instances, in visible blemishes and planes of weakness in the final casting. The presence of aluminum and titanium in many alloys introduces a serious problem in casting such alloys because these elements readily form refractory oxides. Attempts to cast such alloys in shell molds were unsuccessful heretofore due to the folding in or entrapment of this oxide film in the cast metal. The occurrence of a sizable fold results in a plane of weakness since there is little cohesive strength across such a defect. Even very small occlusions of this type are considered as stress concentrators and are to be avoided in any parts, such as turbine buckets, which are subjected to fatigue.

A principal object of my invention, therefore, is to eliminate these aforementioned difliculties by providing a thin-walled sand-resin mold having a carbonized binder which substantially reduces the amount of the volatile constituents which are given off when the mold is contacted by the molten metal during pouring operations. A further object of the invention is to provide a process for forming a rigid, highly permeable shell-type mold by driving off most of the volatiles before the casting metal is poured into the mold by heating or baking the mold in a non-oxidizing atmosphere to carbonize the resin binder Without oxidizing it.

Other objects and advantages of my invention will more fully appear from the following description of a preferred process for forming a shell mold having a carbonized binder.

It 'will be understood that the term mold, as used herein, is generally applied in its generic sense to mean a casting from which includes both molds and cores, this invention in no manner being limited to the former. Similarly, unless otherwise indicated, the Word pattern is used herein as including both mold patterns and core boxes.

In accordance with my invention, therefore, I have found that, in the preparation of sand-resin molds for use in casting readily oxidizable metals, carbonizing these molds in a controlled atmosphere provides stronger molds which produce consistently satisfactory castings of metals of this type. The above-described reduction in mold strength resulting from air curing can be minimized by driving off the volatile constituents by heating the molds in a non-oxidizing atmosphere for A to 1 /2 hours, satisfactory results being obtained with the use of either an inert or reducing atmosphere. If the mold is to be permitted to substantially cool before pouring operations, I have found that it is preferable to have this cooling also occur under the non-oxidizing atmosphere. As a result of this carbonizing procedure, the amount of volatiles given off during pouring operations is substantially reduced and the soundness of the resultant castings is increased considerably.

More specifically, I have found that, among the gases appropriate for use in the formation of these molds having carbonized binders, natural gas or methane, carbon monoxide and helium are readily obtainable gases which are practicable for use and provide superior results. It is preferable, when blanketing the sand-resin molds with natural gas, to use a gas which contains at least 90% methane. The use of such inert or reducing atmospheres prevents oxidation of the binder materials and volatilizes the hydrocarbons contained therein, thereby at least partially carbonizing the resin and eliminating the usual deterioration of the binders. The resultant carbonized binders provide the molds with sufiicient strength and stability during pouring to permit them to be used for many casting operations which heretofore could not be performed with shell molds.

Under certain conditions, I have found it possible to provide the substantially non-oxidizing atmosphere by means of the mold binder itself. This may be done by precluding the admission of air to the mold during the carbonizing treatment and permitting the volatiles driven from the resin binder to function as the nonoxidizing atmosphere required.

As hereinbefore described, prior to stripping the sandresin mold layers from the metallic patterns, the molds are normally baked in a recirculating air oven while in contact with the pattern. The mold and pattern assemblies are retained in these ovens, which are usually heated to a temperature between 300 F. and 1400" F., from a few seconds to approximately five minutes. It is usually desirable to subject the formed thin-walled molds to the above-described carbonizing treatment after they first have been cured by this normal air baking operation. This sequence is preferred inasmuch as it is desirable, in order to prevent distortion of the formed molds, to cure them While in contact with the hot metallic patterns; and heating the molds under a non-oxidizing atmosphere while in contact with the patterns for relatively long periods of time will usually tie up the pattern equipment to too great an extent for practical purposes. The baking cycle under the inert or reducing atmosphere is a comparatively long one, periods of time ranging from A to 1 /2 hours usually being preferable. For the same reason the cured or partially cured molds are preferably stripped from the metal patterns before being subjected to this second heating or baking operation.

Although very low temperatures may be employed, temperatures ranging from 300 F. up to approximately 1400 F. may be advantageously used inasmuch as exposure of the shell molds to such high temperatures even for long periods of time does not result in oxidizing or burning out the binders because of the use of the nonoxidizing gas. As an example, I have found that a carbonizing treatment for approximately one hour at a temperature between 600 F. and 1000 F. is appropriate for a great many applications.

On the other hand, if this tying up of production equipment is not a critical factor in the use of this process, it is possible to eliminate the normal air curing operation and instead to substitute for it the step of baking the cured molds in a non-oxidizing atmosphere while in contact with the metal patterns. The temperatures employed with the non-oxidizing atmosphere heat treatment are substantially the same whether or not the molds have first been subjected to the initial short air-curing operation. When this latter step is omitted, however, it may be desirable to somewhat extend the carbonizing treatment as compared with that period of time which would be used if the molds previously had been baked in a recirculating air oven.

Upon pouring the molten metal into the mold cavities in the usual manner, the hot metal, on coming into contact with the molds, breaks down the carbonized plastic binder to essentially carbon with the generation of only a minimum amount of gases. The very small quantity of gases which may be generated, however, readily escape through the sand-resin shells whose permeability has been increased by my carbonizing process. As the result of this plastic breakdown, the shake-out is readily accomplished.

inasmuch as the above-described molds accurately reproduce pattern details and maintain good dimensional tolerance, they can be used to provide cast metal parts of extremely thin section which can be cast to almost the precise dimensions ultimately desired. The resultant castings have unusually smooth and clean surfaces, true dimensions and a minimum of fin at the mold parting line because of the unusual surface smoothness, high gas permeability and rigidity of these molds. The surfaces of these castings are also free of residual mold material, thereby eliminating the necessity of shot blasting. The cured molds, moreover, have no affinity for water, are completely stable under atmospheric conditions, and may be stored indefinitely. Furthermore, these molds can be produced and processed without objectionable gas or dust formation.

Thus it can be seen that the use of the above-described improved shell molds permits the production of sound precision castings in a wide variety of metals over a wide range of casting temperatures. As hereinbefore indicated, I have found that these molds having carbonized binders are especially useful in casting certain readily oxidizable alloys which recently have been successfully used in forming parts such as turbine buckets and nozzle diaphragm vanes for gas turbines. An example of this type of alloy is a nickel-base alloy containing approximately 1.5% to 3% titanium and 2% to 4% aluminum.

While my invention has been described by means of certain specific examples, it will be understood that the scope of my invention is not to be limited thereby except as defined in the appended claims.

I claim:

1. A process for driving off volatiles from a shell mold formed of a sand-resin mixture prior to pouring operations which comprises heating the mold in a nonoxidizing atmosphere for at least A hour at a temperature of at least 300 F.

2. A method of preparing a shell-type sand-resin mold to reduce the amount of its volatile constituents without oxidizing the carbon contained in the mold binder prior to casting operations which comprises baking said mold in an inert atmosphere for hour to 1 /2 hours at a temperature between 300 F. and 1400 F.

3. A process for carbonizing the thermosetting resin binder in a thin-walled sand-resin mold which comprises heating said mold in a reducing atmosphere for at least 4 hour at a temperature between 300 F. and 1400 F.

and permitting the mold to cool in said atmosphere.

4. A method of increasing the permeability of a shell mold having a thermosetting resin binder which comprises reducing said binder to an essentially high carbon material without the formation of carbon dioxide with the atmosphere by subjecting said mold to a baking operation under a non-oxidizing atmosphere for A1. hour to 1 /2 hours at a temperature up to 1400 F. to drive substantially all the volatile constituents therefrom.

5. A method for preventing a decrease in rigidity of a shell mold having a thermosetting resin binder which comprises reducing said binder and driving off a high percentage of the volatiles from said binder by heating said mold for hour to 1 /2 hours at a temperature of 300 F. to 1400 F. in a reducing atmosphere consisting essentially of at least one gas selected from the group consisting of methane and carbon monoxide and thereafter cooling said mold in said atmosphere.

6. A process of forming a shell mold having high permeability and mold strength prior to metal pouring operations which comprises placing a mixture of a refractory filler material and a thermosetting resin binder into contact with a hot metallic surface for a short time interval, stripping the formed mold from said surface, and thereafter reducing said binder to an essentially high carbon material by baking said mold in a non-oxidizing atmosphere for /4 hour to 1 /2 hours at a temperature between 300 F. and 1400" F.

7. A method of forming a sand-resin shell mold characterized by high strength and a minimum amount of volatile constituents which comprises placing a dry mixture of a major proportion of sand and a minor proportion of a thermosetting resin binder into contact with a heated metallic pattern heated to a temperature between 250 F. and 800 F. for a period of time ranging from a few seconds to one minute, removing excess sand and binder, curing the formed mold while in contact with said pattern by baking for a few seconds to approximately five minutes in a circulating air oven which is heated to a temperature between 300 F. and 1400 F., thereafter stripping the cured mold from said pattern, and finally baking said mold in a non-oxidizing atmosphere.

8. A process for forming a shell-type sand-resin mold characterized by high mold strength and a reduced amount of volatile constituents, which process comprises placing a dry mixture of a major proportion of sand and a minor proportion of a thermosetting resin binder into contact with a heated metallic pattern for a period of time sutii- 25 cient to melt said binder and bond the sand particles together, subsequently curing the formed mold by heating to a temperature in the range between 600 F. and 1000 F. for A hour to 1 /2 hours in at least one reducing gas selected from a class consisting of methane and carbon monoxide, and thereafter permitting the heated mold to cool in said gas.

9. A shell-type sand-resin mold characterized by high mold strength and a minimum of volatile constituents,

said mold comprising a major proportion of dry sand 35 and a minor proportion of a carbonized thermosetting resin binder resulting from driving off a high percentage of the volatile constituents of said binder by baking the mold in a non-oxidizing atmosphere for at least A hour at a temperature of at least 300 F.

10. A shell mold characterized by exceptional permeability, high mold strength and a minimum of oxidizable volatile constituents, said mold comprising a major proportion of sand and a minor proportion of a thermosetting resin binder from which a high percentage of volatile constituents have been driven off by baking in a non-oxidizing atmosphere for 4 hour to 1 /2 hours at a temperature in the range between 300 F. and 1400 F.

11. A method of forming a shell-type sand-resin mold characterized by high strength and a reduced amount of volatile constituents, said method comprising placing a dry molding mixture consisting essentially of 2% to by weight of a thermosetting resin binder and the balance substantially all sand into contact with a hot metal pattern for a short period of time, and thereafter driving 01f a high percentage of volatiles from said binder by heating the formed mold shell in a helium atmosphere for A hour to 1 /2 hours at a temperature between 300 F. and 1400 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,521,614 Valyi Sept. 5, 1950 FOREIGN PATENTS 2,716 Great Britain of 1884 OTHER REFERENCES The Foundry, August 1950, pages 92-96, 206-217, 

4. A METHOD OF INCREASING THE PERMEABILITY OF A SHELL MOLD HAVING A THERMOSETTING RESIN BINDER WHICH COMPRISES REDUCING SAID BINDER TO AN ESSENTIALLY HIGH CARBON MATERIAL WITHOUT THE FORMATION OF CARBON DIOXIDE WITH THE ATMOSPHERE BY SUBJECTING SAID MOLD TO A BAKING OPERATION UNDER A NON-OXIDIZING ATMOSPHERE FOR 1/4 HOUR TO 1 1/2 HOURS AT A TEMPERATURE UP TO 1400* F. TO DRIVE SUBSTANTIALLY ALL THE VOLATILE CONSTITUENTS THEREFROM. 